Processes for preparing individual racemic (2.alpha., 3a.beta., 7a.beta.)-octahydro-lH-indole-2-carboxylic acid and esters species are known.
U.S. Pat. No. 4,350,704 and Blankley et. al. (J. Med. Chem., 30, 992 (1987) disclose that ethyl indole-2-carboxylate, is hydrogenated in a Paar apparatus ##STR2## in a reaction medium containing absolute ethanol and concentrated sulfuric acid with 10% rhodium on charcoal or 10% rhodium on carbon respectively to yield a racemic mixture of ethyl (2.alpha., 3a.beta., 7a.beta.)-octahydro-lH-indole-2-carboxylate. That racemic mixture is resolved through classic multistep procedures i.e. it is then de-esterified, benzoylated, fractionally crystallized as an .alpha.-methylbenzylamine salt and de-benzoylated to yield individual enantiomeric acid species or the mixture is de-esterified, re-esterified, fractionally crystallized as a tartrate salt then de-esterified and basified to yield individual enantiomeric ester species.
U.S. Pat. No. 4,508,729 (hereinafter "the '729 Patent") discloses that the hydrochloride salt of ethyl (2S)-indoline-2-carboxylate is stereospecifically hydrogenated at a high pressure of 50 kg/cm.sup.2 ( 711 psi). The reaction takes place in water using Palladium on charcoal to yield the hydrochloride salt of ethyl [2S-(2.alpha.,3a.beta.,7a.beta.)]-octahydro-lH-indole-2-carboxylate.
The stereospecific hydrogenation of the hydrochloride salt of ethyl (2S)-indoline-2-carboxylate is also disclosed in Tetrahedron Lett., 23(16), 1677 (1982), which authors include all of inventors of the '729 Patent. The reference, however, only superficially discloses details regarding the reaction (i.e., that it occurs in ethanol using palladium on carbon to yield the hydrochloride salt of ethyl [2S-(2.alpha.,3a.beta.,7a.beta.)-octahydro-lH-indole-2-carboxylic ester and that the ester is saponified to the corresponding acid employing a sodium hydroxide solution of water and ethanol).
None of these publications, however, disclose a process for preparing a [2S-(2.alpha., 3a.beta., 7a.beta.)]-octahydro-lH-indole-2-carboxylate ester from an ester precursor by a stereospecific hydrogenation at a pressure below 700 psi. In addition, there is no disclosure of a process for preparing [2S-(2.alpha.,3a.beta.,7a.beta.)]-octahydro-H-indole-2-carboxylic acid or its ester by the hydrogenation of a (2S)-2-carboxyindoline acid precursor.